The objective of the proposed study is to determine the role of certain nuclear proteins in ionizing radiation-induced cell cycle delays in mammalian cells. The hypothesis that certain nuclear proteins play a role in radiation-induced cell-cycle delays will be tested by measuring synthesis of specific nuclear protein(s) in cells blocked in G2 as a result of radiation. Two specific proteins under consideration are Topoisomerase II, a protein involved DNA supercoiling changes, whose synthesis is specifically altered in irradiated cells and a nuclear protein of 55 kD (np 55), an as yet unidentified protein whose synthesis in HeLa cells is stimulated by caffeine. In addition, we will examine the hypothesis that post-radiation cell-cycle delay allows the cell to monitor completeness of DNA repair. Specifically, we will: 1) determine if the relationship of altered Top II synthesis to the radiation-induced G2 block and recovery from it; 2) determine if cell lines with altered Top II levels have altered radiation-induced G2 blocks and/or if altered radiation-induced G blocks are reflected in altered levels of Top II synthesis following radiation. 3) examine the rate of synthesis of other nuclear proteins to determine if np 170 is unique in its synthetic response to radiation; 4) characterize the delayed post-radiation restoration of DNA supercoil rewindability in G2-blocked cells and determine if this effect is related to changes in DNA anchoring and/or the presence of late repairing DNA damage and 5) determine the relationship between the caffeine stimulated synthesis of np 55, caffeine-induced alterations in the synthesis of nucleoid associated proteins np 62 and np 34, caffeine-induced alterations in the size of DNA supercoiling domains and the caffeine-induced reversal of the radiation-induced G2 block.